Abstract：Terahertz filter is an indispensable functional device in terahertz communication, terahertz imaging and terahertz detection systems. According to different classification methods, there are different kinds of filters. Common filters can be divided into high-pass filter, low-pass filter, band-stop filter and band-pass filter according to frequency selection function. In order to achieve the filtering effect in terahertz band, researchers around the world use different structures, materials and control methods to achieve terahertz filters with different functions. However, considering that the designed devices are to be applied to terahertz system, the low cost, simple structure and superior performance of terahertz filters have always been the pursuit of researchers. Fractal concept has developed rapidly in many research fields since it was proposed, but its application in terahertz band is not very common, especially in the design of terahertz functional devices. A novel terahertz bandpass filter is designed and fabricated by introducing the concept of Koch curve in fractal. The filter etches the fractal structure of Koch curve on the metal film. When the terahertz wave is incident perpendicularly to the filter, the narrowband filter in terahertz band is realized. In the process of filter design, the combination of theory and experiment is pursued. Firstly, the Koch curve fractal structure filter model is established in electromagnetic simulation software to calculate, and the feasibility of applying fractal structure to terahertz band filtering is explored. After many calculations, the optimized size and structure are obtained. Then, the Koch curve fractal structure is processed according to the optimized size. Sample of terahertz filter is constructed and measured in terahertz time domain spectroscopy system. The experimental data are obtained and compared with the simulation results. The finite difference time domain method is used to simulate the transmission characteristics of THz bandpass filter with Koch curve fractal structure. The optimized simulation results show that the resonant frequency of the filter is 0.715 THz, the transmission coefficient can reach 0.92, and the bandwidth of -3 dB is 21.9 GHz. The electromagnetic parameters of the THz filter sample are obtained by inversion of the simulated scattering parameters with S parameters, which is analyzed theoretically in the reason of transmission enhancement of terahertz wave at resonance point. Sample of THz bandpass filter with fractal structure of Koch curve optimized is fabricatedby femtosecond laser microfabrication system. The transmission characteristics of the sample are tested by THz time domain spectroscopy system. Frequency domain data are obtained after fast Fourier transform of the experimental time domain data. Frequency domain data are normalized and combined with previous electromagnetic simulation results. By comparing the results, it is found that the experimental results are in good agreement with those obtained by electromagnetic software simulation.
Key words：Terahertz bandpass filter; Fractal of Koch curve; Parameter inversion; Terahertz time-domain spectroscopy system
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